Parsing Utilities

    Overview

    Parsing utilities are a set of functions that helps generate parsing spec for tf$parse_example to be used with estimators. If users keep data in tf$Example format, they need to call tf$parse_example with a proper feature spec. There are two main things that these utility functions help:

    • Users need to combine parsing spec of features with labels and weights (if any) since they are all parsed from same tf$Example instance. The utility functions combine these specs.

    • It is difficult to map expected label by a estimator such as dnn_classifier to corresponding tf$parse_example spec. The utility functions encode it by getting related information from users (key, dtype).

    Example output of parsing spec

    parsing_spec <- classifier_parse_example_spec(
      feature_columns = column_numeric('a'),
      label_key = 'b',
      weight_column = 'c'
    )

    For the above example, classifier_parse_example_spec would return the following:

    expected_spec <- list(
      a = tf$python$ops$parsing_ops$FixedLenFeature(reticulate::tuple(1L), dtype = tf$float32),
      c = tf$python$ops$parsing_ops$FixedLenFeature(reticulate::tuple(1L), dtype = tf$float32),
      b = tf$python$ops$parsing_ops$FixedLenFeature(reticulate::tuple(1L), dtype = tf$int64)
    )
    
    # This should be the same as the one we constructed using `classifier_parse_example_spec`
    testthat::expect_equal(parsing_spec, expected_spec)

    Example usage with a classifier

    Firstly, define features transformations and initiailize your classifier similar to the following:

    fcs <- feature_columns(...)
    
    model <- dnn_classifier(
      n_classes = 1000,
      feature_columns = fcs,
      weight_column = 'example-weight',
      label_vocabulary= c('photos', 'keep', ...),
      hidden_units = c(256, 64, 16)
    )

    Next, create the parsing configuration for tf$parse_example using classifier_parse_example_spec and the feature columns fcs we have just defined:

    parsing_spec <- classifier_parse_example_spec(
      feature_columns = fcs,
      label_key = 'my-label',
      label_dtype = tf$string,
      weight_column = 'example-weight'
    )

    This label configuration tells the classifier the following:

    • weights are retrieved with key ‘example-weight’
    • label is string and can be one of the following c('photos', 'keep', ...)
    • integer id for label ‘photos’ is 0, ‘keep’ is 1, etc

    Then define your input function with the help of read_batch_features that reads the batches of features from files in tf$Example format with the parsing configuration parsing_spec we just defined:

    input_fn_train <- function() {
      features <- tf$contrib$learn$read_batch_features(
        file_pattern = train_files,
        batch_size = batch_size,
        features = parsing_spec,
        reader = tf$RecordIOReader)
      labels <- features[["my-label"]]
      return(list(features, labels))
    }

    Finally we can train the model using the training input function parsed by classifier_parse_example_spec:

    train(model, input_fn = input_fn_train)